(1) Field of the Invention
This invention relates to a process for preparing a polyimide solution. More particularly, it relates to a process for preparing a polyimide solution wherein a tetracarboxylic acid ingredient mainly composed of 3,3',4,4'-biphenyltetracarboxylic acid and an aromatic diamine ingredient mainly composed of 4,4'-diaminodiphenyl ether are polymerized at an elevated temperature in phenol or a halogenated phenolic compound whereby the polymerization of the above-mentioned tetracarboxylic acid and aromatic diamine ingredients is effected while being accompanied by imidation to produce a homogeneous and clear polyimide solution.
(2) Description of the Prior Art
Polyimides have been heretofore prepared by a process wherein polymerization of a tetracorboxylic acid ingredient which an aromatic diamine ingredient for producing a polyamic acid and imidation of the polyamic acid are effected in separate two steps of operation. That is, in the first step of operation, a tetracarboxylic acid ingredient is polymerized with an aromatic diamine ingredient at a relatively low temperature, i.e., not higher than 60.degree. C., in an organic polar solvent thereby to prepare a polyamic acid in a solution form. Thereafter, in the second step of operation, the resulting polyamic acid is converted into the corresponding polyimide in various manners.
Recently, some proposals have been made wherein a tetracarboxylic acid such as 3,3',4,4'-benzophenonetetracarboxylic acid, butanetetracarboxylic acid or 1,4,5,8-naphthalenetetracarboxylic acid and an organic diamine are polymerized at a elevated temperature in an organic polar solvent whereby the polymerization of these acid and diamine reactants is effected while being accompanied by imidation to obtain a solution of a polyimide with an enhanced degree of imidation. However, the polyimides contained in the reaction products produced by these proposed processes exhibit a low molecular weight and/or a low degree of imidation so that the reaction products can be used only in limited applications such as varnishes. Even if the reaction products are formed into shaped articles e.g., films, the resulting films have far poorer mechanical properties than the conventional polyimide films produced by a process wherein the polyamic acid solution obtained by polymerization is shaped into films and, then, the resulting polyamic acid films are imidated.
With respect to the preparation of a polyimide from 3,3',4,4'-biphenyltetracarboxylic acid and an aromatic diamine, it has been proposed in Japanese Laid-open Patent Application No. 113,597/1975 that approximately equimolar amounts of the biphenyltetracarboxylic acid and the diamine are polymerized at an elevated temperature in an organic solvent such as m-cresol and/or xylenol, while water produced by polymerization is removed from the reaction mixture, whereby the polymerization of the acid and diamine reactants is effected while being accompanied by imidation to prepare a polyimide solution. However, it is difficult or even impossible to obtain a reaction product in the form of a homogeneous clear solution containing a high molecular weight polyimide at a high concentration. The reason for this is believed to be that the prepared polyimide having a high molecular weight does not exhibit a high solubility in the m-cresol or xylenol, and thus, at least partially precipitates from the m-cresol or xylenol during the polymerization accompanied by imidation. Therefore, the resulting product as polymerized is neither homogeneous nor clear, and it is difficult to make therefrom polyimide films processing excellent mechanical properties.